Colour wheels are used in a variety of optical devices such as projection-based or other picture generation systems using Digital Light Processing (DLP) technology. The colour wheel comprises a hub portion, which is a disk-shaped body to act as a rotor, when coupled to an associated motor. A optically-active radial portion is attached to or integrated with the outer part of the hub portion. In a colour wheel, the optically-active portion typically includes one or more colour filters, for filtering incident light. These are typically planar glass segments coated with a thin film in order to reflect or transmit light characteristically in a wavelength dependent manner. Multiple colour filters may be provided in different sections of the radial optically-active portion, so that rotation of the colour wheel causes incident light to be affected differently by these different portions For example, this can result in white light being split up into a range of colours and provide colour-sequential illumination when the colour wheel is rotated. Another type of colour wheel uses wavelength conversion materials (phosphors) to generate emission light of a different wavelength from incident excitation light. These can be termed colour phosphor wheels.
In use, colour wheels are rotated at a high speed, typically between 7200 rpm and 14400 rpm. The colour wheel structure is intended to be rotationally symmetric in terms of mass distribution to minimise imbalance, which can result in vibration issues and reduce the product's lifetime. In particular, colour filters of the type discussed above are each mostly formed of a substrate with the same material density (for example, 2.38 g/cm3 for glass). This assists in achieving balance. In such cases, an initial dynamic imbalance should be less than 150 mg@R8.25 mm, especially for an aluminium hub of 30 mm outer diameter after assembly. There are existing approaches to improving imbalance in such a situation, some of which will be discussed below. These will typically be able to correct such a small imbalance.
More recently, a hybrid wheel structure has been developed for use in laser projectors to generate colour sequential illumination with laser speckle reduction. In this structure, one colour filter in the optically active radial portion may be replaced by a translucent diffuser segment, comprising soda-lime glass (density 2.5 g/cm3), which is typically of a higher density than the glass colour filters. It can be significantly more difficult to control the thickness of the diffuser segment, because of problems in etching or polishing this part. The initial imbalance of the hybrid wheel assembly is therefore increased, for example to greater than 300 mg@R8.25 mm in a 60 mm diameter colour wheel.
As noted above, a number of techniques for improving the balance of a colour wheel are known. One possible technique is material removal (from the hub portion), for example as described in US-2007/236816. Material removal may be carried out at multiple planes along the axis of rotation of the colour wheel, in some such approaches. These approaches may allow correction of imbalance up to 150 mg@R8.25 mm. Another balancing approach is material addition, for example as explained in US-2003/035218 (where a balancing substance is added into an annular groove provided on the hub portion), US-2009/021851 (in which balancing weights are positioned in an annular groove and these can be moved to improve balance) and US-2011/063745 (describing a two-dimensional matrix of holes in the hub portion, which can be filled with balancing masses, such as an adhesive filled into one or more of the holes). These approaches cannot generally correct as wide a range of imbalances, for example allowing correction up to 100 mg@R8.25 mm imbalance.
These existing approaches do not offer sufficient ability to correct high imbalances, especially when inhomogeneous materials are used in the colour wheel, as discussed above. Balancing the colour wheel should be achieved with high accuracy, high speed, high cost-efficiency and without increasing the size of the colour wheel significantly. Achieving all these objectives represents a significant challenge.